linux-kernel - Re: [RFC 1/4] asm-generic/uaccess: don't define inline functions if noinline lib/* in use

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Date:   Wed, 15 Jan 2020 14:12:24 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC 1/4] asm-generic/uaccess: don't define inline functions if
 noinline lib/* in use

On Wed, Jan 15, 2020 at 10:08:31AM +0100, Arnd Bergmann wrote:

> > I would suggest that anybody who uses asm-generic/uaccess.h needs to
> > simply use the generic library version.
> 
> Or possibly just everybody altogether: the remaining architectures that
> have a custom implementation don't seem to be doing any better either.

No go for s390.  There you really want to access userland memory in
larger chunks - it's oriented for block transfers.  IIRC, the insn
they are using has a costly setup phase, independent of the amount
to copy, followed by reasonably fast transfer more or less linear
by the size.